Hydrophilic polyurethane resins are prized because of their ability to absorb large amounts of water and to form stable, water-insoluble hydrogels. In the water-swelled state, hydrophilic polyurethanes vary from gel-like to soft and pliable character and in the dry state from soft to hard and machinable quality. The degree of hydrophilicity is a function of the type and proportion of polar groups in the backbone of the polymer, which groups are controlled by appropriate selection of the reactive hydrogen terminated resins or compounds used in the polymer synthesis. These reactive resins and compounds include a variety of polyamines, diols and polyols (polyether or polyester), hydroxyl-containing polycarboxylic acids, and blends of such resins and compounds as well as resins and compounds having mixed functionality. Typical hydrophilic polyurethanes are described in U.S. Pat. Nos. 3,822,238 and 3,975,350. Later patents (such as U.S. Pat. Nos. 4,359,558 and 4,451,635) describe improved versions of the polymers, including polymers based on hydroxyl substituted lactones. The lactone groups can be hydrolyzed to free carboxylic acid groups and the excess hydroxyl groups permit cross-linking of the polyurethanes (U.S. Pat. Nos. 4,156,066 and 4,156,067).
Hydrophilic polyurethanes have a host of uses including catheters and other tubing, low wet-friction coatings, denture liners, cannulae, body implants including corneal prosthesis, contact lenses, dialysis membranes, absorbents, controlled release devices and carriers for drugs and other agents, condoms, swellable fabrics, gauzes, films such as surgical drapes, diaper linings, solubilizing packaging components, water-transmitting coated fabrics, water-swelling caulks, artificial leather, gas filters, oil-resistant shapes, and personal care products, such as hair sprays, nail polishes and the like.
In certain of the foregoing and other applications, increased strength of the polyurethane, including dimensional stability, can be a significant factor. For example, hydrophilic polyurethanes can be used in the manufacture of intravenous catheter tubing because of blood and body fluid compatability, and the ease with which such tubing can be introduced through a needle into a vein. The softening and ease of passage of the tubing of the polyurethane upon hydration (resulting from contact with body fluids) increases the comfort and ease of passage of the tubing. The inside diameter of the tubing also expands upon the hydration, thereby increasing the volumetric flow rate of a fluid being administered through the tubing. Nevertheless, although the tubing can be made sufficiently thin to pass through a needle, the stiffness of the tubing extruded from conventional hydrophilic polyurethanes is insufficient for insertion into veins and other body channels without the needle. It is evident, therefore, that if a hydrophilic polyurethane resin can be modified during processing in such a way that tubing formed of the resin will not only hydrate and thereby swell and soften upon contact with body fluids but will also have sufficient stiffness before such contact, the tubing can be inserted into and moved easily through a vein or other body channel without a needle. This will avoid the discomfort of needles and prevent the tubing from folding back upon itself. The use of stiffened tubing in place of needles also eliminates the risk of contamination sometimes accompanying the use of needles, e.g., transmission of the AIDS virus.
Improved strength manifested by increased stiffness and integrity, has significance also for many other applications of the hydrophilic polyurethanes, particularly when used as carriers in the animal body and other environments for release of active agents such as medicaments. For example, when used as carriers, increased strength enables the polyurethanes to transport a medicament or other agent through body channels more effectively because the polyurethane will not soften and disintegrate as quickly upon contact with body fluids as will conventional hydrophilic polyurethanes. Accordingly, hydrophilic polyurethanes which combine the softening and limited swelling resulting from hydrophilic character with strength and integrity sufficient to enable tubing or other products constructed of the polyurethane to travel more effectively in a closed environment or to support greater loads, will have greater usefulness.
It is well known that water in a polyurethane precursor formulation can cause foaming during the polymerization, and that water in a hydrophobic polyurethane precursor formulation can assist in producing foams ranging from flexible to rigid. Nevertheless, so far as is known there have been no studies on controlling the amount of water relative to the character and amount of diol and NCO/OH ratio in a hydrophilic polyurethane formulation in order to provide a suitable balance of water absorbency, softness and strength in the resulting polymer.
Typical of the technical literature and patents describing the use of water in preparing polyurethane foams is J. H. Saunders et al., Polyurethanes: Chemistry and Technology, Part I, Interscience Publishers, New York (1962) on hydrophobic polyurethanes, and U.S. Pat. Nos. 4,359,558, 4,454,309, 4,454,535, 4,451,635, 3,793,241, 4,083,831, 4,517,326, 2,977,330 and 4,153,777. In the foregoing patents, water is present in the reaction mixture either as a result of the hygroscopic nature of a glycol reactant or as the result of intentionally adding water to the reaction mixture to induce foaming. None of the patents, however, describe the relationships determined in the present invention as controlling the strength of a product hydrophilic polyurethane.